protocol SummaryViewModel {
    var name: String { get }
}

protocol HasSummaryModel {}
extension SummaryViewModel where Self: HasSummaryModel {
    var name: String { return "foo" }
}

protocol DetailsViewModelType: SummaryViewModel {}

class DetailsViewModel: DetailsViewModelType, HasSummaryModel {

}

var sut: DetailsViewModel!

sut = DetailsViewModel()

let context: [String: Any] = ["view_model": sut, "model": "bar"]

let viewModel = context["view_model"] as! SummaryViewModel
let name = viewModel.name